This invention relates to a control system for a parking brake in an earth-moving vehicle. In particular, the invention relates to a control system which provides for automatic application of the vehicle parking brake in the event the vehicle engine is shut down or if there is an emergency situation as when there is a loss of oil pressure in the engine or a loss of electrical power, the control system at the same time enabling the operator to set the brake manually at any other time.
Many earth-moving vehicles employ safety or parking brake systems which include a brake-actuator that is normally spring-biased to a brake-setting position. Fluid under pressure from the hydraulic system of the vehicle is admitted to the brake-actuator to overcome the force of the brake-setting spring and thereby release the brake. Subsequent relief of such pressure from the actuator then enables the brake-setting spring to reapply the brake. An example of such a system is disclosed in U.S. patent application Ser. No. 435,113, entitled "Parking Brake Control System," filed on Jan. 21, 1974 by John W. Bridwell and Norman J. Hodge, of common assignment herewith.
Such parking brake systems should have a variety of capabilities. First, such systems should be able to supplement the primary braking system of the vehicle to ensure vehicle safety in the event of emergency conditions. For example, if the engine should die during operation of the vehicle, there will be a loss of hydraulic pressure required for operation of the primary brake system. In such event the parking brake system should cause an automatic application of the parking brake to bring the vehicle to a halt.
Secondly, the parking brake system should also be usable for normal parking situations wherein the parking brake is to be set to prevent vehicle movement when the vehicle is not in operation. In order to safeguard against operator forgetfulness, the parking brake should be set automatically when the engine is turned off.
Many times the vehicle may be at a standstill with the operator wishing to leave it for a short time without shutting down the engine. The parking brake system should have the capability of allowing the operator to apply the parking brake manually while the engine is operating.